where the second expression emphasizes the generalization of the BCS relation. The constants provided best fits to the computational data. The “effective Coulomb repulsion” \(\mu _{eff}^*=\mu ^*(1+0.62\lambda )\)  is defined (without physical justification) to emphasize the similarity to the BCS equation (1). The \(1+\lambda \)  factor in the numerator is often assigned to the strong-coupling electron mass renormalization by EP coupling.
The mass enhancement is well known to increase the density of active states at the Fermi level by 1+\(\lambda \) , but in this expression the effect in the numerator is to decrease T\(_c\) , compensated somewhat by the \(\lambda \mu ^*\)  term in the denominator. Flores-Livas et al. have reproduced the model that gives this analytic result, for \(\mu ^*(\lambda =0)\)  and a different expression for the prefactor.[1]}
In McMillan's time the experimental Debye frequency \(\theta \)  was the most accessible measure of the frequency spectrum, however McMillan recognized that using values from \(\alpha ^2F\)  (such as the second moment \(\omega _2\) ) would be more representative.
